Promotion of CO Oxidization and Inhibition of NO Formation

Feb 2, 2011 - with the increase of air stoichiometric ratio and CO concentration in off-gas. ... above 70% at T = 2073 and 2273 K, and gaseous iron sp...
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Promotion of CO Oxidization and Inhibition of NO Formation by Gaseous Iron Species during High-Temperature Off-Gas Combustion Sen Li and Xiaolin Wei Institute of Mechanics, Chinese Academy of Sciences, No. 15 Beisihuanxi Road, Beijing, China, 100190 ABSTRACT: The promotion of CO oxidization and inhibition of NO formation by gaseous iron species were analyzed using the Sandia SENKIN program. It is shown that the relative ratio of CO oxidization dramatically varies with combustion time at early burning stage in the presence of iron species, and all peak values are greater than 2.4. The relative ratio of CO oxidization decreases with the increase of air stoichiometric ratio and CO concentration in off-gas. The circulation reactions of Fe-FeO/FeO2-Fe achieve the catalytic effect on CO oxidization. Gaseous iron species can greatly inhibit NO formation, NO reduction ratio can reach above 70% at T = 2073 and 2273 K, and gaseous iron species can effectively inhibit NO formation when combustion temperature is not higher than 2273 K during the off-gas combustion. There are O2 competitive reactions between thermal NO formation and Fe oxidization, and high chemical activity of iron species inhibits thermal-NO formation.

1. INTRODUCTION A large amount of high-temperature off-gas is produced during oxygen converter steelmaking, the major compositions of off-gas are CO and CO2, CO concentration varies from 15 to 70%, and off-gas temperature can reach 1900 K.1 In the meantime, off-gas entrains a large amount of fine dusts (Fe, FeO, Fe2O3, etc.) and the amount of the dusts entrained by off-gas is about 80-150 g/m3.2 The high-temperature off-gas is a precious valuable fuel. It is often discharged into the cooling stack to be combusted, and the peak temperature of flame can reach above 2000 °C where CO2 may decompose and NOx is significantly formed.3 CO emission concentration can reach above 2000 mg/m3 which is always over emission standards (300 mg/m3 in China).4,5 CO is a toxic gas which is dangerous to human health. High CO emission results in not only atmospheric pollutant but also fuel loss. NOx is a known precursor to the formation of ozone and acid rain, and it can react with volatile organic compounds to form photochemical smog.6 Iron oxide catalyst is widely used either as single metal oxide or as a mixed oxide in industrial processes, and iron oxide exhibits intermediate activity in the complete oxidation of methane and carbon monoxide.7-9 NO reduction by iron species has been studied in fluidized bed, and the results indicate that iron or its oxides cause a chemical reduction of NO in the presence of CO.10 It has been discovered that injection of iron-containing compounds into the combustion and reburning zones of conventional gas- and coal-fired combustors can increase the efficiency of NOx reduction by up to 20% in comparison with that which can be achieved by basic reburning.11 During the combustion of high-temperature off-gas entraining a large amount of dust containing iron and iron oxides, gaseous iron species is present at T > 2073 K, and it has effects on CO oxidization and NO formation. A detailed chemical kinetic mechanism for gaseous iron-species inhibition of flames, iron pentacarbonyl (Fe(CO)5) mechanism, has been introduced,12 and modeling with the mechanism supports the premise that the inhibition is primarily a gas-phase phenomena. Many researchers r 2011 American Chemical Society

used the Fe(CO)5 mechanism to investigate the influence of Fe(CO)5 on combustion flames, and the mechanism was validated by experimental results.12-16 Understanding the behaviors of promotion of CO oxidization and inhibition of NO formation by gaseous iron and iron oxides at high-temperature off-gas combustion is important for the control of CO and NO emissions. There are no reports in the literature of NO by ironcontaining compounds in steelmaking off-gas combustion conditions. The goal of this work is to gain better understanding of the mechanisms of the promotion of CO oxidization and inhibition of NO formation by gaseous iron species. In the paper, the combustion of off-gas in the presence of gaseous iron species was simulated using the Sandia SENKIN program, and the promotion of CO oxidization and inhibition of NO formation by gaseous iron species were analyzed.

2. KINETIC MODEL The chemical kinetic behavior of reactants was modeled with the Sandia SENKIN program of CHEMKIN subroutines.17,18 The program solves the conservation equations for mass and energy. It can calculate the temporal evolution of mole fractions of species for a homogeneous mixture in a closed reactor and it can account for finite-rate elementary chemical reactions and perform kinetic sensitivity analysis. Therefore, the SENKIN model is appropriate to investigate the mechanisms of CO oxidization and NO reduction by gaseous iron species in the process of high-temperature off-gas combustion in a closed chamber. The reaction pathway flux analysis is performed using MixMaster (a Python program that is part of the Cantera suite), which is based on a conserved scalar approach to reaction fluxes.19 The chemical kinetic model of off-gas combustion is from Linteris et al.20 In the mechanism, the CO/H2/O2 system is from Received: November 17, 2010 Revised: January 3, 2011 Published: February 02, 2011 967

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Figure 1. Initial concentration of gaseous iron species in high-temperature off-gas.

Figure 2. Concentrations of CO, gaseous Fe, and FeO during off-gas combustion at the initial CCO = 25%, SR = 0.7, and T = 2273 K.

Allen and Yetter,21 based upon that from Kim et al.,22 while the gaseous iron species mechanism is from Rumminger et al.23 The CO/H2/O2 submechanism has been extensively validated by experimental data in ref 22, and the iron submechanism has been validated using flame speed data and diffusion flame extinction data.16,23-25

steel-smelting bath during converter steelmaking process, it can be considered that the gaseous Fe species is in an equilibrium saturated state in the high-temperature off-gas, thus the initial concentration of gaseous iron species calculated by chemical equilibrium is reasonable. The results of chemical equilibrium calculation indicate that the gaseous compositions of iron-containing species are Fe and FeO; the concentrations of gaseous Fe and FeO increase with the increase of temperature; with the increase of CO concentration, the concentration of gaseous Fe increases, but FeO concentration is not effected. Moisture content of air supplied for combustion is 0.6 wt %. Air stoichiometric ratio (SR) represents the fuel-air mixture composition: SR ¼ ðactual air suppliedÞ=ðstoichiometric air demand of fuelÞ

3. SIMULATION CONDITIONS In the combustion simulation of high-temperature off-gas containing gaseous iron species by Sandia SENKIN model with the mechanism from Linteris, it is a case for an adiabatic constant pressure system (P = 1.01 bar). The initial compositions of offgas are CO and CO2, and the oxidizer is air. In the paper, CCO is the initial CO concentration in off-gas, and CCO,0 is the initial CO concentration in the gas mixture of off-gas and air during combustion. The promotion of CO oxidization and inhibition of NO formation by gaseous iron species are investigated at CCO = 25% and 50%. According to the analysis data of dust sampled at the cooling stack outlet (72.0% Fe2O3, 0.80% SiO2, and 9.04% CaO, 120 g/ m3),2 using chemical equilibrium calculation by HSC chemistry 7.0,26 the initial concentrations of gaseous iron species in hightemperature off-gas were calculated, as shown in Figure 1. Because the high-temperature off-gas originates from the high-temperature

ð1Þ In the paper, the influences of SR on CO oxidization and NO formation are investigated, and SR ranges from 0.3 to 1.2.

4. RESULTS AND DISCUSSION 4.1. Catalytic Effect of Gaseous Iron Species on CO Oxidization. Figure 2 compares the variations of CO concentrations

during off-gas combustion in the presence and absence of gaseous 968

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Figure 3. Influence of SR on the relative ratio of CO oxidization at CCO = 25%.

iron species at CCO = 25%, SR = 0.7 and T = 2273 K, and the concentrations of gaseous iron species are also provided in the presence of gaseous iron species during off-gas combustion. CO concentration in the presence of gaseous iron species decreases faster than that in the absence of gaseous iron species, indicating that gaseous iron species during off-gas can greatly promote CO oxidization. To estimate the contribution of gaseous iron species to CO oxidization, the relative ratio of CO oxidization is defined as (RCO) Rco ¼

CCO,0 -CCO,2 CCO,0 -CCO,1

ð2Þ

where CCO,0 is the initial CO concentration in the gas mixture of off-gas and air during off-gas combustion; CCO,1 is the CO concentration in the absence of gaseous iron species during offgas combustion; CCO,2 is the CO concentration in the presence of gaseous iron species during off-gas combustion. RCO > 1 means that the presence of gaseous iron species during off-gas combustion is conducive to CO oxidization, and the higher RCO, the more significant the catalytic effect of gaseous iron species on CO oxidization. In Figure 3, the relative ratio of CO oxidization is shown as a function of time at CCO = 25% and SR = 0.3-1.2. The simulation results indicate the following: at early burning stage (t < 3 μs), RCO dramatically varies with combustion time, it has a peak value when combustion time is 0.5 μs or so, all of the peak values of RCO are greater than 2.7, and increasing SR makes RCO decrease; at t > 5 μs, RCO moderately decreases with combustion time and tends to 1, and SR has little effect on RCO; RCO at T = 2473 K is higher than that at T = 2273 K. In Figure 4, the relative ratio of CO oxidization is shown at initial CCO = 50% and SR = 0.3-1.2. When comparing RCO at COCO,0 = 25% and 50% (see Figures 3 and 4), increasing CCO makes Rco decrease. To understand the influence mechanism of gaseous iron species on CO oxidization, the normalized sensitivity of CO concentration toward reactions is calculated. The sensitivity analysis coefficients are obtained through perturbation of the pre-exponential factor A in the Arrhenius equation of reactions, and the normalized sensitivity coefficient (Sj) is defined as Dln c ð3Þ Si ¼ Dln ki

Figure 4. Influence of SR on relative CO oxidization ratio at CCO = 50%.

where c is CO concentration, and ki is the pre-exponential factor A in the Arrhenius equation of the reaction i. The value of the sensitivity coefficient indicates a positive or negative influence on c by ki, a positive value means that the value of c increases as ki increases and a negative value means that the value of c decreases as ki increases. The reason for the high value of this coefficient is that the reaction is the key chain branching step and strongly promotes CO oxidation. As known from Figure 5, the normalized sensitivity coefficients of reactions R1R1-R4R4 are the top 4, meaning the four reactions are most important for CO oxidization. All of the reactions R1R1-R4R4 involve gaseous iron species, and these reactions comprise the catalytic CO oxidization. Fe þ O2 ¼ FeO þ O

ðR1Þ

FeO2 þ O ¼ FeO þ O2

ðR2Þ

FeO þ CO ¼ Fe þ CO2

ðR3Þ

Fe þ O2 ¼ FeO2

ðR4Þ

Net reaction : O2 þ 2CO ¼ 2CO2 Therefore, during the combustion of high-temperature off-gas, gaseous iron species have great catalytic effect on CO oxidization. 969

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The peak values of the sensitivity coefficient of reactions R1R1-R4R4 coincide in time at the early stage of off-gas combustion. These reactions are responsible for the rapid increase of the relative ratio of CO oxidization at early burning stage in Figures 3 and 4, indicating that gaseous containing-iron species during off-gas can greatly promote CO oxidization. The primary reaction pathways for CO and iron species are shown in Figure 6. Figure 6a indicates that the most important reaction of CO oxidization in the absence of iron species is

OH is crucial to CO oxidization. OH radical is formed to accelerate CO oxidation in the presence of even a small quantity of hydrogen-containing material.27 For off-gas combustion, offgas contains virtually no hydrogen-containing material, OH only originates from H2O vapor supplied by combustion air stream, and OH is principally obtained by the following mechanisms:

ðR5Þ

Therefore, during off-gas combustion, H2O vapor supplied by combustion air stream is very important, and the absence of H2O may make the oxidation of CO extremely slow. Figure 6b illustrates the reaction pathways for CO oxidization in the presence of gaseous iron species. It indicates that there are three reaction pathways of CO oxidization: (1) FeO þ CO = Fe þ CO2; (2) OH þ CO = H þ CO2; (3) O þ CO = CO2. The first reaction pathway is the most important pathway and accounts for 72% contribution for CO oxidization. Here, a portion of Fe is converted first to FeO2 and then to FeO, the remainder of the Fe is directly converted to FeO, and FeO participates in CO oxidization to be reduced into Fe. In the series of reactions, and the circulation reactions of Fe-FeO/FeO2-Fe achieve the catalytic effect on CO oxidization, which predominates CO oxidization. The secondary pathway of CO oxidation is the same as that in the absence of iron species mentioned in Figure 6a; it accounts for 26% contribution for CO oxidization in the presence of iron species which accounts for 84% in the absence of iron species, and the pathway originates from the reaction of CO and H2O vapor supplied by combustion air

OH þ CO ¼ H þ CO2

Figure 5. Sensitivity of CO concentrations toward the most important reactions at CCO = 25%, T = 2273 K, and SR = 0.7.

H2 O þ O f 2OH

ðR6Þ

H2 O þ H f OH þ H2

ðR7Þ

Figure 6. Schematic diagram of reaction pathways for CO and iron species at CCO = 25% in off-gas, T = 2273 K, SR = 0.7, and t = 1 μs. The percentages presented in some paths represent the contribution of that path to the consequent species. Large arrows represent the important paths for both consumed and produced species. 970

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stream. The third reaction pathway CO oxidation accounts for only 1% contribution for CO oxidization, and it is negligible. Based on the above analysis of sensitivity and reaction pathways, the influences of SR, temperature, and CCO on the relative ratio of CO oxidization are discussed. Figures 3 and 4 indicate that increasing SR makes the relative ratio of CO oxidization decrease at the early combustion stage. To investigate the reason, the schematic diagrams of reaction pathways for CO oxidization

at different SR are presented in Figures 7. The results indicate that, with the increase of SR, the contribution rate of R3 (FeO þ CO = Fe þ CO2) for CO oxidization decreases, but the contribution rate of R5 (OH þ CO = H þ CO2) for CO oxidization increases. When SR increases from 0.3 to 1.2, the increasing air supplied dilutes the concentration of gaseous iron species in the gas mixture of off-gas and air, the initial concentration Fe species decreases from 285 to 196 ppm, but the initial concentration H2O vapor supplied by air stream increases from 48 to 133 ppm. Since the reaction rate of R3 is larger that of R5, the decrease of the initial concentration Fe species with SR is not conducive to CO oxidization. The catalytic effect of gaseous iron species on CO oxidization reduces, thus the relative ratio of CO oxidization deceases with the increase of SR at early stage of off-gas combustion (see Figures 3 and 4). In Figure 3, the reason for the increase of the relative ratio of CO oxidization with the increase of temperature is as follows: the reaction rate coefficients have exponential relationship with combustion temperature, increasing temperature strengthens catalytic effect of gaseous iron species on CO oxidization, and then the relative ratio of CO oxidization at T = 2473 K is higher than that at T = 2273 K (see Figure 3). In Figures 3 and 4, the reason for the decrease of the relative ratio of CO oxidization with the increase of CCO is as follows: the off-gas combustion with high-concentration CO needs more air provided, and a large amount of combustion air results in the decrease of the concentration of gaseous iron species, the catalytic effect of gaseous iron species on CO oxidization is weakened, and then the relative CO ratio of oxidization deceases at early stage of off-gas combustion with the decrease of CCO as compared between Figure 3 and Figure 4. 4.2. Catalytic Effect of Gaseous Iron Species on NO Formation. Figure 8 compares the variations of NO concentration during off-gas combustion in the presence and absence of gaseous iron species at CCO = 25%, SR = 0.7 and T = 2273 K. NO concentration in the presence of gaseous iron species is much lower than that in the absence of gaseous iron species, indicating that gaseous iron species can greatly inhibit NO formation. To estimate the effect of gaseous iron species on NO formation, NO reduction ratio is defined as RNO

Figure 7. Schematic diagrams of reaction pathways for CO oxidization at CCO = 25% in off-gas, T = 2273 K, and t = 1 μs.

Figure 8. NO concentration during off-gas combustion in the presence and absence of gaseous containing-iron species at initial CCO = 25% in off-gas, SR = 0.7, and T = 2273 K.

RNO ¼

CNO,2 -CNO,1  100% CNO,2

ð4Þ

Figure 9. Influence of temperature and SR on relative CO oxidization ratio at CCO = 25%. 971

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Figure 10. Schematic diagram of reaction pathways for NO formation at CCO = 25%, T = 2273 K, SR = 1.2, and t = 1 μs. The percentages presented in some paths represent the contribution of that path to the consequent species. Large arrows represent the important paths for both consumed and produced species.

where CNO,1 is the NO concentration in the presence of gaseous iron species, and CNO,2 is the NO concentration in the absence of gaseous iron species. Figure 9 shows NO reduction ratio of off-gas in the presence of gaseous iron species at SR = 0.7 and 1.2. NO reduction ratio can reach above 70% at T = 2073 and 2273 K, but it is less than 50% at T = 2473 K. NO reduction ratio slightly increases with the increase of SR. The results indicate that gaseous iron species can effectively inhibit NO formation when combustion temperature is not higher than 2273 K during the off-gas combustion. Figure.10 shows NO formation pathways during the off-gas combustion in the absence and presence of gaseous iron species, and the results indicate that major reaction pathways of NO formation in the absence and presence of gaseous iron are similar, where N2/N mainly reacts with O2 and O to form NO. To understand NO formation mechanism, by using the rate of production (ROP) analyses, NO production rates of reactions were calculated (see Figure 11). The top three reactions during high-temperature off-gas combustion in the absence and presence of gaseous iron are the same, and they are following reactions: ðR8Þ N2 þ O ¼ N þ NO N þ O2 ¼ NO þ O

ðR9Þ

N þ CO2 ¼ NO þ CO

ðR10Þ

and oxygen gases in the air during combustion, especially at high temperatures. Reaction R8R8 has a very high activation energy due to the strong trip bond in the N2-molecule, and is thus sufficient at high-temperature. The formation of thermal-NO is determined by a set of highly temperature-dependent chemical reactions known as the extended Zeldovich mechanism. The presence of CO2 has a significant effect on NO formation by reaction R10R10, the high concentration of CO2 makes more NO formation. At initial stage of off-gas combustion, NO production rates of the reactions R8R8 and R9R9 rapidly increase because of high O2 concentration, and then the production rates decreases with combustion time because of the decrease of O2 concentration (see Figure 11). As compared with the NO production rates during off-gas in the absence of gaseous iron, the NO production rates are noticeably low in the presence of gaseous iron, indicating that gaseous iron species plays a key role in the NO reduction. To investigate the reasons for the inhibition mechanism of NO formation by gaseous iron species, the normalized sensitivity of NO concentration toward reactions involving gaseous iron species reactions is calculated, and the top three sensitivities of NO concentration toward reactions involving gaseous iron reactions are shown in Figure 12. The sensitivity analysis reflects the relative importance of different reactions with respect to the NO mole fraction. Figure 11 indicates that the reactions of iron species greatly influence NO formation, and the top three reactions are the

Reactions R8R8 and R9R9 are thermal-NO or ZeldovichNO; thermal-NO is produced from the reaction of nitrogen 972

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Figure 11. Rate of production (ROP) analyses of NO formation at CCO = 25%, T = 2273 K, and SR = 1.2.

species during the combustion of converter steelmaking off-gas was performed using the Sandia SENKIN program of CHEMKIN subroutines. In the presence of iron species, at early burning stage (t < 3 μs), RCO dramatically varies with combustion time, all of the peak values of RCO are greater than 2.4, and increasing SR makes RCO decrease. These reactions comprise the catalytic CO oxidization: Fe þ O2 d FeO þ O, FeO2 þ O = FeO þ O2, Fe þ O2 d FeO2, FeO þ CO = Fe þ CO2. The circulation reactions of Fe-FeO/FeO2-Fe achieve the catalytic effect on CO oxidization. Gaseous iron species can greatly inhibit NO formation, and NO reduction ratio can reach above 70% at T = 2073 and 2273 K. Gaseous iron species can effectively inhibit NO formation when combustion temperature is not higher than 2273 K during the off-gas combustion. There are O2 competitive reactions between thermal NO formation and Fe oxidization, and high chemical activity of iron species inhibits thermal-NO formation.

Figure 12. Sensitivity of NO concentrations toward the most important reactions at T = 2273 K, SR = 1.2, and CCO = 25%.

following:

’ AUTHOR INFORMATION Fe þ O2 ¼ FeO þ O

ðR1Þ

Fe þ O2 ¼ FeO2

ðR3Þ

FeO2 þ O ¼ FeO þ O2

ðR4Þ

Net reaction : 2Fe þ O2 ¼ 2FeO Reactions R1R1, R3R3, and R4R4 play a key role not only in CO oxidization but also in NO formation. O2 is very important for thermal-NO formation. As mentioned above, reactions R1R1-R4R4 have the catalytic effect of CO oxidization, and they play a role of O2 transfer for CO oxidization. The circulation reactions of Fe-FeO/FeO2-Fe predominate CO oxidization, but thermal NO greatly depends on O2. Thus, there are O2 competitive reactions between thermal NO formation and Fe oxidization. High chemical activity of iron species easily reacts with O2/O (see Figure 6b), thus thermal-NO formation is inhibited. In short, the gaseous iron species promotes CO oxidation and inhibits NO formation in the combustion of off-gas.

5. CONCLUSION In the paper, a numerical simulation of the promotion of CO oxidization and inhibition of NO formation by gaseous iron

Corresponding Author

Telephone: þ8610-82544231. Fax: þ86 10 82544231. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support by National Natural Science Foundation of China (50976123) and the Knowledge Innovation Program of the Chinese Academy of Sciences (KGCX2-YW-321) is acknowledged. ’ NOMENCLATURE CCO = initial CO concentration in off-gas (%) CCO,0 = initial CO concentration in the gas mixture of off-gas and air during combustion (%) CCO,1 = CO concentration in the absence of gaseous iron species during off-gas combustion (%) CCO,2 = CO concentration in the presence of gaseous iron species during off-gas combustion T = combustion temperature (K) SR = air stoichiometric ratio RCO = the relative ratio of CO oxidization RNO = NO reduction ratio (%) Sj = the normalized sensitivity coefficient 973

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